Document 6Br0xbX0jL5gpem6gKGkRzbkE

202 CHAPTER 9 1950 Guide material, or some vapor resistant coating may be applied to the inner or warm surface of the wall. In applying vapor resistance to a wall, there are certain fundamental principles which should be followed. First, the vapor barrier should be placed as near to the warm surface of the wall as practicable. Second, it should be continuous with no direct openings through the barrier. If membrane barriers are used back of the plaster of interior finish, the joints should be lapped over some solid framing member, and not between the studs or in similar places. Usually a two-inch lap over a framing member will make a sufficiently tight joint when the interior finish is applied. Such a lap, however, without backing would not be adequate. All open ings for electrical fixtures and joints around window and door casings should be oarefully sealed. Since, in applying vapor barriers the primary purpose is to prevent water vapor from entering the warm side of the wall, the barrier, in order to be effective, must be placed near the warm side, and all joints must be suffi ciently tight to prevent direct leakage of the vapors. The limiting per meability for a material which .may be considered as a barrier, will depend upon the requirements. For ordinary residential work, it has generally been considered that a material having a permeability of one grain of mois ture per (sq ft) (hr) (in. Hg of vapor pressure) difference across the barrier is adequate. There are cases, however, in residential construction where a barrier having a permeability of 1.00 would not be sufficient, and there are also many industrial applications in which a very much higher vapor re sistance is required. The best time to vapor-proof a building is during its construction. After the building is completed the remedies are limited largely to operational control and surface treatment of the structure. Ventilation of Structure Condensation difficulties may often be eliminated by lowering the dew point temperature or the relative humidity by ventilation. It is much more practicable to apply ventilation in open spaces than it is in interior parts of the structure. For a wall construction it is far better to seal the warm surface so that the vapor cannot enter, than it is to try to ventilate the vapor out of the wall once it has entered. Wherever possible, it is prefer able to eliminate the moisture at its source rather than to rely on ventila tion. Condensation on the interior surface of cold attic walls may be eliminated by ventilation. However, in new construction and in other places where practicable, it is far better to use vapor barriers and other means to prevent the vapors from entering the attic space. Ventilation is often uncertain in its effect and, furthermore, it is a source of some heat loss. Where ven tilation is used for attics or other parts of a building, precautions must be taken to see that the air is adequately distributed throughout the space to be ventilated. No fixed amount can be given for the ventilation required, but for the ordinary home with gravity attic ventilation, the inlet and out let openings should be well distributed and the total area of each should be one-quarter square inch per square foot of floor. These openings should be distributed with due regard to the type of construction, outside wind veloc ities, and all factors which affect the circulation of air. The conditions are so varied that no hard and fast rules can be set down which will cover all cases. The best defense against condensation on attic walls and other similar surfaces, is to prevent the vapors from entering these spaces. Ven- -Heat Transmission Coefficients of Building Materials 203 tilation is a precaution, but not the best direct solution of most condensa tion problems. REFERENCES Standard Method of Test for Thermal Conductivity by Means of the Guarded Hot Plate, sponsored by A.S.H.V.E., A JS.T.MA JS.R.E., and N.R.C. and approved as a Tentative Code by A.S.H.V.E. and A.S.T.M. in 1942 (.4.S/J'.M. designation C-177-42T, Approved 1945). Heat Transmission Through Building Materials, by F. B. Rowley and A. B. Algren (University of Minnesota, Engineering Experiment Station Bulletin No. 8, p. 11). Radiation and Convection from Surfaces in Various Positions, by G. B. Wilkes and C. M. F- Peterson (A.S.H.V.E. Transactions, Vol. 44, 1938, p. 513). Radiation Corrections for Basic Constants Used in the Design of All Types of Heating Systems, by B. F. Raber and F. W. Hutchinson (A.S.H.V.E. Transactions, Vol. 51,1945, p.213). 1 Radiation and Convection Across Air Spaces in Frame Construction, by G. B. Wilkes and C. M. F. Peterson (A.S.H.V.E. Transactions, Vol. 43,1937, p. 351). Thermal Test Coefficients of Aluminum Insulation for Buildings, by G. B. Wilkes, F. G. Hechler and E. R. Queer (A.S.H.V.E. Transactions, Vol. 46,1940, p. 109). 7 Effect of Studs and Joists on Heat Flow Through Frame Walls and Ceilings, by Paul D. Close (Heating, Piping and Air Conditioning, October, 1943, p. 529). A.S.H.V.E. Research Report No. 1213--Heat Los3 Through Basement Walls and Floors, by F. C. Houghten, S. I. Taimuty, Carl Gutberlet and C. J. Brown (A.S.H.V.E. Transactions, Vol. 48,1942, p. 369). Measurements of Heat Losses from Slab Floors, by R. S. Dill, Wm. C. Robinson and H. E. Robinson (National Bureau of Standards, Building Materials and Struc tures Report BMS 103). BIBLIOGRAPHY AH.H.V.E. Research Reports: No. 852--Effects of Air Velocities on Surface Coefficients, by F. B. Rowley, A. B. Algren and J. L. Blackshaw (A.S.H.V.E. Transactions, Vol. 36, 1930, p. 123). No. 895--Wind Velocity Gradients Near a Surface and Their Effect on Film Con ductance, by F. C. Houghten and Paul McDermott (A.S.H.V.E. Trans actions, Vol. 37, 1931, p. 301). No. 914--Surface Coefficients as Affected by Direction of Wind, by F. B. Rowley and W. A. Eckley (A.S.H.V.E. Transactions, Vol. 38,1932, p. 33). No. 915--Conductivity of Concrete, by F. C. Houghten and Carl Gutberlet (A.S.H.V.E. Transactions, Vol. 38, 1932, p. 47). <; No. 964--The Heat Conductivity of Wood at Climatic Temperature Differences, by F. B. Rowley (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 329). No. 996--Insulating Value of Bright Metallic Surfaces, by F. B. Rowley (AH.H.V.E. Transactions, Vol. 40,1934, p. 413). No. 1026--Thermal Properties of Concrete Construction, by F. B. Rowley, A. B. Algren and Clifford Carlson (A.S.H.V.E. Transactions, Vol. 42, 1936, p. 33). No. 1048--Thermal Properties of Concrete Construction, by F. B. Rowley, A. B. Algren and Robert Lander (A.S.H.V.E. Transactions, Vol. 43, 1937, p. 33). Insulating Effect of Successive Air Spaces Bounded by Bright Metallic Surfaces, by E. W. Schad (A.S.H.V.E. Transactions, Vol. 37,1931, p. 285). Thermal Conductivity of Wood, by J. D. MacLean (A.S.H.V.E. Transactions, Vol. 47,1941, p.323). The Specific Heat of Thermal Insulating Materials, by G. B. Wilkes and C. O. Wood (A.S.H.V.E. Transactions, Vol. 48, 1942, p. 493). Heat Loss Studies in Four Identical Buildings to Determine the Effect of Insula tion, by D. B. Anderson (A.S.H.V.E. Transactions, Vol. 48,1942, p. 471). Effect of Ceiling Insulation upon Summer Comfort, by T. D. Phillips (National Bureau of Standards, Report BMS52, July 1,1940).